1. Field of the Invention
The present invention relates to the field of d.c./d.c. voltage converters for supplying a load by maintaining the voltage across the load at a predetermined value. The present invention more specifically applies to step-down type converters meant to supply, by means of a battery, a mobile device. In particular, the present invention applies to supplying a mobile phone from a rechargeable battery.
2. Discussion of the Related Art
D.c./d.c. converters can be essentially divided into two categories. A first category involves switched-mode power supplies and a second category involves linear regulators.
FIG. 1 shows an example of a conventional converter of switched-mode power supply type (SMPS). Such a converter includes two P-channel and N-channel MOS transistors, respectively, MP and MN, connected in series between two terminals A, B, that are used to apply a d.c. input voltage Vbat provided, for example, by a rechargeable battery 2. Terminal B represents the circuit ground. The midpoint 3 of the series connection of transistors MP and MN is connected to a first terminal of an inductance L. A second terminal of inductance L is directly connected to an output terminal S meant to supply a load Q at a predetermined voltage Vout. A storage capacitor C, generally a chemical capacitor of high value, is connected between terminal S and the ground. A decoupling capacitor C' is further generally connected between the second terminal of inductance L and the ground. It generally is a ceramic capacitor of low value. Inductance L is associated with a recovery diode D connected between its first terminal and the ground. A pulsewidth modulation (PWM) control block 1 controls transistors MP and MN to provide an output voltage Vout at the desired predetermined value. Block 1 receives a signal FB taken at the midpoint of a series connection of resistors R1 and R2 connected between terminal S and the ground. Block 1 further receives a clock signal (not shown) and a capacitor Cin is generally connected in parallel on the battery between terminals A and B. The operation of such a converter is well known and will not be further described.
FIG. 2 shows an example of conventional diagram of a positive voltage linear regulator. Such a regulator essentially includes an amplifier 4 that controls a power component MP that supplies a load Q at a predetermined voltage Vout. A rechargeable battery 2 is connected between input terminals A and B of the regulator, terminal B forming the ground of the assembly. Load Q is connected between an output terminal S of the regulator and the ground. The power component is generally formed of a MOS transistor, for example, with a P channel, to minimize, with respect to the use of a bipolar transistor, the so-called waste voltage, that is, the voltage drop between terminals A and S of the regulator, and to save the current "entering" through the base of a bipolar transistor. The source of transistor MP is connected to terminal A while its drain forms terminal S. A decoupling capacitor C' is generally connected between terminal S and the ground, and a capacitor Cin is generally connected between terminals A and B in parallel on rechargeable battery 2. Amplifier 4 includes a first inverting input connected to a terminal R on which is applied a reference voltage Vref. A second non-inverting input of amplifier 4 is connected to the gate of transistor MP to modify, according to the error voltage between the inverting and non-inverting inputs, the gate-source voltage of transistor MP, and thus maintain voltage Vout at reference value Vref.
The choice between a switched-mode power supply and a linear regulator depends on the application and, in particular, on the type of rechargeable battery used.
Indeed, the evolution of the discharge of rechargeable batteries is different according to their type. For example, cadmium-nickel (Ni-Cd) type batteries have an abrupt discharge characteristic, that is, the voltage that they provide remains substantially steady before abruptly dropping. Conversely, batteries of lithium-ion (Li-ion) type have a smooth discharge characteristic, that is, the voltage that they provide progressively decreases along their use.
This is particularly disturbing in the specific application to mobile phones. Indeed, in such an application, several phones (for example, 8) share a same communication channel. As a result, the needs in current of a given phone are not constant. It is generally required to switch from a full charge mode to a mode of almost no current in less than 10 .mu.s. This raises no problem if the battery voltage is sufficiently high with respect to the output voltage. Conversely, if the input voltage is low, then this constraint cannot be respected, since the current slope is linked to inductance L (FIG. 1). To respect this constraint, the switched-mode power supply should be operated at much higher frequencies than their usual frequencies on the order of 200 kHz.
Another disadvantage of a step-down type switched-mode power supply is that it has a higher waste voltage than a linear regulator. In practice, a switched-mode power supply requires at least 3 volts of supply voltage for an output voltage of 2.7 volts.
Further, in a mobile phone, the switched-mode power supply has two operating modes. A first operating mode is meant for periods of high current consumption by the load. In this mode, the control pulse trains have a fixed frequency. In such an operating mode, the internal consumption of the switched-mode power supply is on the order of 1 mA. A second operating mode (generally referred to as "PFM in SKIP MODE") is an operating mode where, while remaining synchronous with the fixed frequency of the first mode, clock cycles are skipped. Thus, in the second operating mode, not only the pulsewidth varies, but also the frequency. This operating mode is meant for periods of lower current surge by the load and results in a lower internal power consumption on the order of 100 .mu.A. However, the frequency decrease of the pulse train introduces a noise problem since the frequency is generally in the audio band used by telephony. It is thus necessary to use additional filters to avoid disturbances.
This would thus lead to a preference for linear regulators, in particular, for lithium-ion batteries. However, a linear regulator has other disadvantages.
A disadvantage is that the efficiency of such a regulator is inversely proportional to the input voltage. Thus, for a lithium-ion battery, a very poor efficiency is obtained when the battery is in full charge. Further, since the power consumption of the linear regulator is substantially constant whatever the current consumed by the load, this consumption is linked to the maximum current for which the regulator is provided, and is then particularly high in low current surge periods.
With nickel-cadmium batteries having a nominal voltage which is higher and not too far from the regulator output voltage, a linear regulator is generally used since the decrease in the voltage of the battery has a slope close to zero until the time when it abruptly drops.